The invention relates to superconducting magnetic coils.
As is known in the art, the most spectacular property of a superconductor is the disappearance of its electrical resistance when it is cooled below a critical temperature T.sub.c.
Below T.sub.c and a critical magnetic field, a superconductor can carry a electrical current density up to a critical current density (J.sub.c) of the superconductor. The critical current density is the current density at which the material loses its superconducting properties and reverts back to its normally conducting state.
Superconductors may be used to fabricate superconducting magnetic coils such as solenoids, racetrack magnets, multipole magnets, etc., in which the superconductor is wound into the shape of a coil. When the temperature of the coil is sufficiently low that the HTS conductor can exist in a superconducting state, the current carrying capacity as well as the magnitude of the magnetic field generated by the coil is significantly increased.
Typical superconducting materials include niobium-titanium, niobium-tin, and also copper oxide ceramics such as members of the rare-earth-copper-oxide family (i.e., YBCO), the thallium-barium-calcium-copper-oxide family (i.e., TBCCO), the mercury-barium-calcium-copper-oxide family (i.e., HgBCCO), and BSCCO compounds containing lead (i.e.,(Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10). Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 (BSCCO (2223)), performs particularly well because its superconductivity and corresponding high current density characteristics are achieved at relatively high temperatures (T.sub.c =115 K).
Referring to FIG. 1, in fabricating such superconducting magnetic coils, the superconductor may be formed in the shape of a thin tape 5 which allows the conductor to be bent around the diameter of a core. The thin tape is fabricated as a multi-filament composite superconductor including individual superconducting filaments 7 which extend substantially the length of the multi-filament composite conductor and are surrounded by a matrix-forming material 8, which is typically silver or another noble metal. Although the matrix forming material conducts electricity, it is not superconducting. Together, the superconducting filaments and the matrix-forming material form the multi-filament composite conductor. In some applications, the superconducting filaments and the matrix-forming material are encased in an insulating layer (not shown). The ratio of superconducting material to matrix-forming material is known as the "fill factor" and is generally less than 50%.
A magnetic coil can be wound with superconducting tape using generally one of two approaches. In the first approach, known as layer winding, the superconductor is wound about a core with turns being wound one next to another until a first layer is formed. Subsequent layers are then wound on top of previous layers until the desired number of layers are wound on the core.
In an other approach, known as pancake winding, the superconductor tape is wound one turn on top of a preceding turn thereby forming a plane of turns perpendicular to the axis of the coil. In applications where a series of pancake coils are to be used to form a coil, the pancake coils can be wound as double pancakes.
In some applications, a superconducting magnetic coil assembly using pancake coils (whether single or double) may include several coils, coaxially disposed along the length of the coil assembly. The individual coils are interconnected using short lengths of superconducting wire or ribbon made from the superconducting materials of the type described above, for example, copper oxide ceramic.